1. Field of the Invention
The present invention relates to a noise reduction circuit and an image processing device.
2. Description of the Related Art
To reduce a noise included in an image, heretofore, it has been common to use an average filter which takes an average of a signal value of a denoising target pixel (hereinafter, referred to as a “pixel of interest”), and signal values of pixels (hereinafter, referred to as “neighboring pixels”) in the neighborhood of the pixel of interest. However, the average filter has a problem of blurring a significant edge (contour, pattern) included in an image. To solve the problem, ε-filter, bilateral filter, and the like have been proposed. Each filter gives a weight (calculates a filter coefficient) to each neighboring pixel according to the absolute value of the difference (hereinafter referred to as a difference absolute value) between the signal value of the pixel of interest and the signal value of the neighboring pixel. For ε-filter, see, for example, Kondo Hiroyuki, Hanari Kenji, Shimizu Katsuhide, and Arakawa Kaoru, “Colored Face Image Processing by Vector ε-Filter: Removal of Wrinkles” in Proceedings of the IEICE General Conference, 1998, D-11-143 (March, Heisei 10 (1998) (Non-patent document 1)). For bilateral filter, see, for example, Tomasi, C. and Manduchi R., “Bilateral filtering for gray and color images” in Proc. Of International Conference on Computer Vision, IEEE 1998, pp. 839-846 (Non-patent document 2).
Here, a neighboring region is defined by the number of horizontal and vertical filter taps. For example, there are eight neighboring pixels excluding the pixel of interest in the filter of 3×3 taps.
ε-filter is expressed as follows:
                                                                        y                ⁡                                  (                  n                  )                                            =                                                ∑                                      k                    =                                          -                      N                                                        N                                ⁢                                                      a                    k                                    ⁡                                      [                                                                  x                        ⁡                                                  (                          n                          )                                                                    -                                              f                        ⁢                                                  {                                                                                    x                              ⁡                                                              (                                n                                )                                                                                      -                                                          x                              ⁡                                                              (                                                                  n                                  -                                  k                                                                )                                                                                                              }                                                                                      ]                                                                                                                          =                                                ∑                                      k                    =                                          -                      N                                                        N                                ⁢                                                      a                    k                                    ⁢                                                            x                      ′                                        ⁡                                          (                                              n                        -                        k                                            )                                                                                                                              Equation        ⁢                                  ⁢                  (          1          )                                                              x            ′                    ⁡                      (                          n              -              k                        )                          =                  {                                                                                          x                    ⁡                                          (                                              n                        -                        k                                            )                                                        :                                                                                                                                                                    x                        ⁡                                                  (                          n                          )                                                                    -                                              x                        ⁡                                                  (                                                      n                            -                            k                                                    )                                                                                                                          ≤                  ɛ                                                                                                                          x                    ⁡                                          (                      n                      )                                                        :                                                                                                                                                                    x                        ⁡                                                  (                          n                          )                                                                    -                                              x                        ⁡                                                  (                                                      n                            -                            k                                                    )                                                                                                                          >                  ɛ                                                                                        Equation        ⁢                                  ⁢                  (          2          )                    where x(n) indicates an input signal, y(n) indicates an output signal, and 2N indicates the number of taps ak, the summation of which is set to 1.
ε-filter requires nonlinear circuits (difference-absolute-value calculation, filter coefficient calculation) as many as the number obtained by subtracting 1 from the number of pixels existing in the neighboring region. This produces a problem that, as the number of filer taps becomes larger, the circuit size increases. Against this problem, for example, Japanese Patent Application Laid-open Publication No. 2000-295497 (JP-A No. 2000-295497) discloses a method using a filter configured of only nonlinear circuits for an average value of all the pixels within taps, and a signal value of a pixel of interest.
Furthermore, in order to decrease the occurrence of a phenomenon referred to as a “ghost” in which a fake contour appears at a position spaced a little away from an edge when the method in JP-A No. 2000-295497 is employed, Japanese Patent Application Laid-open Publication No. 2006-14024 (JP-A No. 2006-14024), for example, discloses a method which is simplified by limiting, to 0 or 1, weights given to neighboring pixels without decreasing the number of nonlinear circuits.
However, in the case of the method using a difference absolute value of the average value of all the pixels within taps as in JP-A No. 2000-295497, some combination of the frequency band of a signal included in an input image signal, and the number of filter taps may cause a blur in an image after denoising, and a ghost around edges in the image.
Furthermore, in the simplified method described in JP-A No. 2006-14024, a weight calculation process (difference calculation, threshold comparison) is performed for all the pixels included in filter taps. Accordingly, in the case where the number of taps is large, and where the band of a signal included in an input image signal is narrow, the process is redundant.